Compressors of refrigerating devices, such as a domestic refrigerator with a freezer, recently employ hydrocarbon refrigerant, such as R600a, which is a natural refrigerant having an ozone depleting coefficient of zero and a small global warming coefficient.
A conventional compressor disclosed in Japanese Patent Laid-Open Publication No. 11-311457 rotates at a low frequency at its start at a low ambient temperature, at which a large amount of refrigerant dissolves in lubricant. At the start, the lubricant is discharged while bubbles generated by vaporization of the refrigerant are sucked. When the compressor rotates at a constant frequency, a discharged amount of the lubricant decreases. As a result, an amount of the lubricant in the compressor is maintained, and this prevents lack of the lubricant supplied to sliding components.
A controller of controlling the conventional compressor will be described hereinafter. FIG. 10 is a sectional view of a conventional refrigerator. FIG. 11 shows a refrigerating cycle of the conventional refrigerator. FIG. 12 is an electrical schematic diagram of the conventional refrigerator. FIG. 13 shows a change of an operating frequency of the conventional compressor.
As shown in FIG. 10, reciprocating compressor 10 is placed at the lower and rear section in refrigerator 1. The reciprocating compressor includes lubricant, motor 11, and a mechanism driven by motor 11. Those elements are accommodated in an airtight container. Compressor 10 includes a lubricating mechanism (not shown) formed of a centrifugal pump therein. The airtight container accommodates compressor motor 11, a crank mechanism (not shown), and a piston (not shown). A rotary shaft of motor 11 is linked to the piston via the crank mechanism which converts a torque of motor 11 into a linear reciprocating force and transmits the force to the piston. Refrigerant in the airtight container is compressed by the reciprocating force of the piston.
As shown in FIG. 11, compressor 10 is coupled to condenser 13 via pipe 12a. Condenser 13 is coupled to capillary tube 14 via pipe 12b. Capillary tube 14 is coupled to evaporator 15 via pipe 12c. Evaporator 15 is coupled to a suction inlet of compressor 10 via pipe 12d. The foregoing structure forms refrigerating cycle 16 having refrigerant sealed therein.
As shown in FIG. 12, motor 11 is a three-phase DC brush-less motor including a stator formed of a stator core having coils 11a of phases U, V and W wound around the core, and a rotor formed of a rotor core and permanent magnets rigidly mounted to the rotor core. Motor 11 is coupled to inverter 18 shown in FIG. 12.
Inverter 18 includes main controller 22 implemented mainly by a micro-computer. Main controller 22 determines an operating frequency of motor 11 in response to an electrical signal corresponding to room temperature T. The electrical signal is supplied from room-temperature sensor 28, such as a thermister, placed at refrigerator 1.
An operation of the controller of the refrigerant compressor will be described hereinafter. Upon inverter 18 outputting a power at a predetermined frequency to motor 11, compressor 10 compresses the refrigerant, then the refrigerant discharged from compressor 10 circulates through condenser 13, capillary tube 14, and evaporator 15 in this order.
A large amount of refrigerant generally dissolves into lubricant in compressor 10 at a low ambient temperature. At this moment, if compressor 10 is activated at a high frequency, the dissolving refrigerant evaporates at once, thereby producing bubbles intensely.
In order to prevent the bubbles from being produced, when main controller 22 of inverter 18 detects the relation of reference temperature T0≧ambient temperature T, the controller raises the frequency of the power applied to motor 11 from 0 Hz (the motor halts) to 30 Hz, which is a minimum frequency, within about 3 seconds, then holds the frequency at 30 Hz. This operation allows the refrigerant dissolving in the lubricant to evaporate gradually, hence preventing the bubbles from being intensely produced. Then, the lubricant is prevented from being discharged from compressor 10 together with the refrigerant, and the lack of lubrication can be prevented.
However, when compressor 10 operates at the minimum frequency (30 Hz) at a low ambient temperature, in the conventional controller, the refrigerant dissolving in the lubricant evaporate little. Therefore, at low ambient temperature at which the large amount of the refrigerant dissolves in the lubricant, when the compressor operates from the frequency of 30 Hz to an ordinary operation at a high rotation speed, a large amount of the refrigerant evaporates at once, hence producing the bubbles intensely. Compressor 10 then compresses the refrigerant together with the bubbles including a large amount of lubricant, thereby generating an abnormal noise. Simultaneously to this, an amount of lubricant is discharged from compressor 10, and then, a lack of lubrication and an obstacle to lubrication occur in compressor 10.
It has taken a long period of time for the refrigerant to dissolve in the lubricant. Therefore, the above phenomenon often occurs at an initial starting, i.e., when a refrigerating device is energized for the first time. This phenomenon often occurs at a start after a defrosting operation since the refrigerant in condensed form returns into compressor 10 from evaporator 15.
For a combination of hydrocarbon refrigerant, such as R600a, recently introduced and lubricant made from mineral oil, a saturation solubility of the refrigerant to the lubricant depends extremely on a pressure. At the start of the compressor, the pressure in the airtight container is reduced, hence producing the bubble intensely.